Support HiDPI (retina) displays in docs

[mdboom]

For 2.0, it would be nice to show off the new styles in the docs by having proper hi-resolution image support.

It may be that using SVG by default is now sufficient, since browser support for SVG has come a long way since we first put together the docs. Alternatively, we can provide images at twice the current resolution, scaled down (which probably won't look as good on traditional low-DPI displays).

[Tillsten]

+1 for svg, would also solve #5097